# TBR Physics Question

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#### HopefulOncoDoc

##### Full Member
10+ Year Member
Hey everyone I have a question on TBR Example 7.2b in the 2nd Physics book.

A swimmer is at the bottom of a 5-meter pool, where the gauge pressure (pgh) is approximately 0.5Patm. If the swimmer rises to the surface, by how much does the total pressure change?

A. The total pressure increases by 33%
B. The total pressure increases by 66%
C. The total pressure decreases by 33%
D. The total pressure decreases by 66%

So using Ptotal = Patm + Pgauge

Initially
Ptotal = 1.5 atm

Final
Ptotal = 1 atm

So 1/1.5 = 66% so shouldn't the pressure decrease by 66%? What am I doing wrong? The answer is C btw. Thanks!

Everthing you did seems right.

Except. 1/1.5 = 66% of the initial remains after swimmer rises to the surface. The other 33% is gone...thats is by how it is reduced.

Nice. Thanks!

Nice. Thanks!

This is an easy process of elimination problem. You know that when the guy rises to the surface, the pressure will decrease(After all, the guage pressure is from the water).
So this eliminates anything about the pressure *increasing*. Goodbye A and B.
Now, did it decrease by 66% or by 33%? What was the total pressure? 1.5atm. What is it at the water surface? 1atm. Just looking at it, you can tell that less than half was reduced, not more. So it's 33%.
You did the math correctly. But you took the information for granted and just went with it. Did it make sense for the pressure to drop by 2/3rds or 1/3rd?

This is an easy process of elimination problem. You know that when the guy rises to the surface, the pressure will decrease(After all, the guage pressure is from the water).
So this eliminates anything about the pressure *increasing*. Goodbye A and B.
Now, did it decrease by 66% or by 33%? What was the total pressure? 1.5atm. What is it at the water surface? 1atm. Just looking at it, you can tell that less than half was reduced, not more. So it's 33%.
You did the math correctly. But you took the information for granted and just went with it. Did it make sense for the pressure to drop by 2/3rds or 1/3rd?

Excellent reasoning! That is exactly what you need for the MCAT! As a side note, I'm so glad to see you answering chapter 7 questions, because that means you worked through all of the BS questions in the first few chapters. I have to admit that there is some excess b.s. in the first couple of chapters that you have to sift through to get into the optimal frame of mind for attacking questions.